Hi,
NEXT - PREV has always been problematic in Virtuemart. CleanMart is not responsible to generate the links of those buttons. CleanMart only style the buttons by adding the CSS Class names.. That's all.
If any of your product stays in multiple categories, then NEXT-PREV buttons become unpredictable (simply because VM doesn't know the "next or prev product" in what category).
Also, make sure to sort your product properly in the Virtuemart Backend >> Products page. Make sure they don't have all 0 for the sorting. Because NEXT-PREV are sorted by the "Default Ordering".
Regards